Biot Equation
Definition
The fundamental equation of polarimetry: [α]_T^λ = α / (c × l), relating observed rotation to specific rotation, concentration, and path length; established by Jean-Baptiste Biot experimentally in the 1820s.
- Formula
- [alpha]_T^lambda = alpha / (c x l)
- Relates
- Specific rotation, observed rotation, concentration, path length
- Originator
- Jean-Baptiste Biot, 1820s
- Field
- Polarimetry, forensic chemistry
Common questions
Why does the Biot equation require standardising temperature and wavelength?+
Specific rotation is temperature and wavelength dependent, so the same compound gives a different reading under different conditions. Reporting specific rotation with its temperature (T) and wavelength (lambda) subscripts lets a forensic result be compared meaningfully against reference values.
What forensic question does the Biot equation help answer?+
It lets an examiner convert an observed rotation angle from a polarimeter into a concentration of an optically active substance, such as a sugar or a controlled substance, once the compound's specific rotation and the sample's path length are known.
Related terms
- Chirality
- The property of a molecule that cannot be superimposed on its mirror image; a chiral molecule and its mirror image are enantiomers...
- Dextrorotatory (+)
- Rotating the plane of polarisation clockwise (to the right) as seen facing the light source; designated (+) or d-; the d-methamphetamine and...
- Enantiomeric Excess (Ee)
- A measure of chiral purity: ee (%) = (R - S)/(R + S) x 100, where R and S are the measured...
- Enantiomers
- Two molecules that are non-superimposable mirror images of each other; they have identical physical properties (melting point, boiling point, solubility) except for...
- Invert Sugar
- The equimolar mixture of glucose and fructose produced by hydrolysis (inversion) of sucrose; laevorotatory overall because fructose's negative rotation exceeds glucose's positive...
- Laevorotatory (-)
- Rotating the plane of polarisation anticlockwise (to the left) as seen facing the light source; designated (-) or l-; l-methamphetamine and natural...
- Laurent Half-Shade Polarimeter
- A classical polarimeter design that uses a half-wave plate covering half the aperture to create two adjacent half-fields; the null point (equal...
- OIML R 139
- International Organisation of Legal Metrology Recommendation R 139 (2014) governing metrological requirements for polarimeters used in official testing; specifies maximum permissible error...
- Optical Rotation (α)
- The angle in degrees through which a chiral solution rotates the plane of polarisation of a plane-polarised light beam; measured by rotating...
- Saccharimeter
- A polarimeter calibrated in International Sugar Degrees (°Z) rather than angular degrees; used in the sugar industry for purity determination under the...
- Specific Optical Rotation ([α])
- The standardised optical rotation of a substance at a defined temperature and wavelength, calculated from the Biot equation: [α] = α /...